1. Field of the Invention
The present invention relates to a mobile telecommunication system, and in particular to a mobile telecommunication system in which a sound signal is transmitted after being coded and compressed according to the ADPCM (Adaptive Differential Pulse Code Modulation) scheme.
2. Description of the Related Art
Conventionally, as a mobile telecommunication system in which sound data are transmitted after being coded and compressed according to the ADPCM scheme, there is a personal-handyphone-system transmission system (hereafter referred to as "PHS transmission system") which is disclosed in JP-A-7-30645, for example. The PHS transmission system is used in the communication scheme for transmitting sound and image signals using the S interface of an ISDN (Integrated Services Digital Network). For example, a compressed signal produced by encoding and compressing the sound signal according to the ADPCM scheme is assigned to a first channel 15, which consists of four bits among eight bits of B1 channel (B1-ch) as shown in FIG. 3A, to be transmitted as a signal of 32 kbps. An uncompressed signal (non-sound signal) containing other information is assigned to a second channel 16, which consists of the remaining bits of the B1 channel and eight bits of B2 channel (B2-ch) as shown in FIG. 3B, to be transmitted as a signal of 96 kbps.
In the PHS transmission system, however, it is necessary to use two channels (i.e. the B1 and B2 channels) in order to simultaneously transmit and receive the compressed signal (sound signal) and the uncompressed signal (non-sound signal). Furthermore, in the case where the data amount of the uncompressed signal (non-sound signal) is small, the circuit utilization factor between PHS terminals reduces.